1. Field of the Invention
The disclosed invention relates generally to integrated circuits and silicon chip technology, and more particularly, but not by way of limitation, the packaging of optoelectronic devices in a dense and integrated manner.
2. Description of the Related Art
Optical transceivers are increasingly used to enable data communication between and within computers. An optical transceiver can transmit and receive data using optical fiber rather than electrical wire. The optical fiber can be used to facilitate the transfer of information in light beams, rays or pulses along solid transparent fibers or cables. One of the major advantages of optical technology is its high transfer rate.
However, today's optical transceiver packages are bulky, complex and expensive. The performance of high-end computers systems continues to improve as the number of processing cores (and their speed) is increased. This increase in the number of processors requires a corresponding improvement in the system's interconnect bandwidth. Today's high-end supercomputer systems are being built with hundreds of thousands of individual optical fibers to transmit this data, at considerable expense.
The communication bandwidth between computers and within a computer is playing an increasing role in a system's overall performance. The trend towards multi-core processors and multiple processors per machine requires an increase in communication between processors and between a processor and its memory. Electrical data links perform well over short distances, but they reach a limit as the link distance and frequency increases. Optical data links over fiber are capable of high speed communication with low loss over large distances. However, current optical transceivers are bulky and expensive compared with their electrical counterparts.
Currently, a standard 12 channel optical transceiver can be mated with a 12 fiber ribbon cable, with each fiber containing only 1 core. However, a single core in the optical fiber increases the space that is used by the arrangement and can be expensive.
Recently it has been proposed to use multiple graded index and/or single mode cores inside a single optical fiber to save space and reduce cost. What is needed is a means to couple the light into and out of these new multi-core fibers in a simple and low cost manner. In further detail, what is also needed is a means to couple light from OE (opto-electronic) devices, such as VCSELs (vertical cavity surface emitting lasers) and photodiodes, into a multi-core fiber.
There is a related art method to couple multi-core fiber to a VCSEL/PD (vertical cavity surface emitting lasers/photodiode) array. In this case the multi-core fiber contains 4 cores. The multi-core fiber is butt coupled to a 4 element VCSEL array.
However, the multi-core fiber must be close to the active device of the VCSEL array for best coupling, but not touch the VCSEL array. If the fiber touches the VCSEL array, then the VCSEL array may be damaged. There is possible damage during assembly, and the optical fiber is not easily connectorized.
Another problem relates to the NA (numerical aperture) of the VCSEL that may be greater than a typical NA of a multi-mode fiber. Therefore, given this mismatch in NAs, a coupling loss may occur, leading to a lower overall optical coupling efficiency. Therefore, the VCSEL NA not matching, but instead overfilling the fiber NA, leads to lower efficiency. These limitations reveal that the fiber-to-VCSEL “butt” coupling method is less than optimal.
In another arrangement, a long focal length dual lens optical coupling arrangement shows poor optical performance when used with a multi-core fiber. The long focal length dual lens optical coupling arrangement has been previously used to couple light between single core optical fibers rather than a multi-core fiber. A relatively long focal length lens can successfully image the core from one fiber to another.
However, when a multi-core fiber is used, the cores are offset from the center axis of the lens. The object (core) offset causes light from the source to miss a portion of the collimating lens, leading to a loss of light (lower efficiency) and potential cross-talk between neighboring fibers.
Another arrangement includes a short focal length dual lens optical. In this case the focal length of the collimating lenses or telecentric lens pair is short so that the light from the offset source does not miss the collimating lens. However, given the lens short focal length and the offset source, it is difficult to focus the light into the multi-core fiber. For example, the light ray misses the core of the multi-core fiber. The offset sources requires tight lens manufacturing tolerances. The alignment tolerances of optoelectronic elements (OE), such as VCSEL, in the X, Y, and Z axis are critical. The light rays at the multi-core fiber are abberated and overfill the cores, possibly leading to a drop in coupling efficiency and a potential for cross-talk between optical channels.
Accordingly, it is desirable to provide an apparatus and method to optically interconnect a multi-core optical fiber with a compact optical transceiver in order to improve optical coupling performance, and to have a simpler package and lower cost. Moreover, concerning the desire for improved optical coupling performance, there is a desire to reduce or avoid a loss of light and potential for cross-talk between optical channels.
In addition, it is also desirable to provide a means to reduce the number of optical transceivers needed by a computer system by using multi-core optical fibers to increase the number of optical channels while maintaining or reducing the optical transceiver's package size.